Finned tube with indentations

ABSTRACT

A tube used for heat transfer has adjacent fins extending from an outer surface of the tube with a channel between the fins. The fins include a roof formed over the channel, and holes penetrate the roof into the channel. The fin, including the roof, is monolithic with the tube body. Helical ridges are formed on a tube inner surface, and the tube body includes an indentation in the outer surface which extends the tube body inner surface towards a tube axis.

This invention claims priority to Chinese Patent Application NumberZL200720068218.6, which was filed on Mar. 27, 2007.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The current invention describes finned tubes used for heat transfer,such as the tubes used in shell and tube heat exchangers.

2. Description of the Related Art

Finned tubes have been used for heat transfer for many years. Heat flowsfrom hot to cold, so heat transfer is accomplished by conducting heatfrom a warmer material to a cooler material. There is also heat givenoff when a material condenses from a vapor to a liquid, and heat isabsorbed when a liquid vaporizes or evaporates from a liquid to a vapor.When finned tubes are used for heat transfer, the warmer material is oneither the inside or the outside of the tube and the cooler material ison the other side. Usually the tube allows for the transfer of heatwithout mixing the warmer and cooler materials.

For cooling purposes, a cooling medium can be a liquid such as coolingwater flowing through a shell and tube heat exchanger, or it can be agas such as air blown over a finned tube. Similarly, a heating medium isusually either a liquid or a gas. Finned tubes are sometimes usedinstead of relatively smooth tubes because finned tubes tend to increasethe rate of heat transfer. Therefore, a smaller heat exchanger withfinned tubes may be able to transfer as much heat in a given applicationas a larger heat exchanger with relatively smooth tubes. The design offinned tubes affects the rate of heat transfer and sometimes the tubesare designed differently for specific heat transfer applications. Forexample, finned tubes used for condensation tend to have differentdesigns than finned tubes used for evaporation.

Examples of the prior art include finned tubes with helical fins formedon an outer surface of the tube. The tops of the fins have at least onegroove to divide the fin top into at least two parts, thus forming a “Y”shape along the length of the fin. The fins can also be notched acrossthe fin, and then helical indentations are formed by pressing into thetube outer surface. The fins are broken where the indentations areformed, and beads are formed on a tube inner surface where the fin ispressed down into the tube body. There are a plurality of beads formedon the inner surface along an imaginary line corresponding to where theindentation in the tube is formed.

Finned tubes also include fins formed to promote boiling on the outersurface. The fins are deformed at the top to essentially close off thechannels defined between adjacent fins, except the closed off channelsare open to the outside through pores penetrating into the channels. Thepores can be of varying sizes, and there can be more than one sized poreon a single tube. There are a wide variety of finned tubes forevaporation which include various permutations of closed off channelsbetween adjacent fins, with some sort of hole or pore penetrating intothe closed off channels.

Some finned tubes are produced by attaching fin material to a relativelysmooth tube so the fins are not formed from the material of the tubebody. This increases the area available for heat transfer, which doesimprove heat transfer rates, but the interface between the fin and thetube does cause some resistance to heat flow. The fins attached to thetube can extend radially from a tube axis so they stand straight up fromthe tube, but they can also be curved or bent in various ways to improveheat transfer.

Finned tubes are often used in evaporators, such as those used in airconditioners. Most air conditioning evaporators are a flooded type ofevaporator, where the finned tube is submerged in a pool of liquidrefrigerant. The surface of a tube in a flooded evaporator is constantlywet with the refrigerant, and evaporated gas bubbles through the liquidpool to escape. It is also possible to use a drip evaporator, where theliquid refrigerant is circulated and dripped or spayed on top of thetubes, and the tubes are not immersed in liquid. This allows for the useof less refrigerant, and tends to be more energy efficient. Keeping allareas of the heat transfer tube wet improves the boiling efficiency of adrip evaporator, and tube designs which improve the wetted area of atube are beneficial.

There are many designs of finned tubes in existence, but changes whichimprove heat transfer are still possible.

BRIEF SUMMARY OF THE INVENTION

A tube used for heat transfer has adjacent fins extending from an outersurface of the tube with a channel between the fins. The fins are formedfrom the material of the tube outer surface, so the fins are monolithicwith the tube body. The fins include an upper portion which is deformedinto a roof over the channel, and there are holes penetrating the roofinto the channel. Helical ridges are formed on a tube inner surface, andan indentation is defined in the tube body outer surface that extendsthe tube body inner surface towards a tube axis.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a side view of a tube with indentations.

FIG. 2 is the enlarged portion from the circle labeled “2” in FIG. 1.FIG. 2 shows a side sectional view of a tube with indentations with thedetails of the fins and ridges not shown.

FIG. 3 is the enlarged portion from the rectangle labeled “3” in FIG. 2.FIG. 3 shows a side sectional view of a finned tube.

FIG. 4 is a side sectional view of a finned tube.

FIG. 5 is a perspective view of a portion of a finned tube.

FIG. 6 is a side partial sectional view of a finned tube withindentations.

FIG. 7 is a side view of one embodiment of a tube with an indentation.

FIG. 8 is a side view of a second embodiment of a tube withindentations.

FIG. 9 is a side view of a third embodiment of a tube with indentations.

FIG. 10 is a side partial sectional view of heat exchanger.

FIG. 11 is a side partial sectional view of an arbor forming a finnedtube with material from a fin in front of a tooth on the notching disc.

FIG. 12 is a side partial section view of an arbor forming a finned tubeshowing a notching disc tooth forming a notch in a fin.

DETAILED DESCRIPTION

The finned tube of the current invention is intended to be used for heattransfer, and primarily for phase change on the tube outer surface. Thistube is designed to especially enhance boiling or evaporation, but itcould also be used for condensation or non-phase change heat transfer.The tube is designed to promote boiling on the tube outer surface with aheating medium, such as a liquid, flowing inside the tube. The heatingmedium is cooled by the evaporation on the tube outer surface. The tubeis often utilized in the construction of shell and tube heat exchangers,but other uses are possible.

Heat Transfer Principles

The following discussion is directed towards evaporation, andparticularly towards evaporation using a drip type evaporator. Thecurrent invention could also be used for other heat transferapplications as well, and this discussion is not intended to limit thescope of the invention.

When heat is transferred from one material to another material, thelarger the temperature difference between the two materials, the fasterthe rate of heat transfer. This basically means if you want to heatsomething up faster, put it against a hotter surface, and if you want tocool something off faster, put it against a colder surface. This is truefor a conductive material or an insulating material.

A refrigerant on the tube exterior absorbs the heat of vaporization asit changes from a liquid to a gaseous state, and this heat ofvaporization is ultimately absorbed from the liquid flowing inside thetube. This cools the liquid in the tube. The design of the fins on thetube outer surface increase heat transfer by several differentmechanisms. The fins increasing surface area of the tube, and the finsform boiling nucleation sites which serve to promote boiling. The moresurface area on a condensing tube, the more rapid the flow of heat. Whenfins are formed on a tube it increases the surface area of the tube,which serves to increase the rate of heat transfer across the tube.Other deformations in the tube outer surface which increase surface areawill also tend to increase the rate of heat transfer.

When heat is transferred from a liquid inside a tube to a vapor formedby boiling a liquid on the tube outside, the heat transfer is consideredin several distinct steps. The same basic steps apply when heat istransferred through a barrier, such as a tube wall, between any twomediums with different temperatures. The first step is transfer of heatfrom the liquid inside the tube to the tube inner surface. Liquidflowing through the tube tends to form an essentially stagnant layer onthe tube inner surface. In laminar flow, there is a second liquid layernext to the stagnant layer that is moving slowly, or just sliding by thestagnant layer. Then there is a third layer next to the second, which ismoving a little bit faster, and so on such that the fastest flowingliquid is furthest from the tube wall. These layers tend to insulate thetube wall and hinder the flow of heat.

In turbulent flow, there is still a stagnant layer next to the insidetube wall, but the rest of the liquid is flowing and mixing together asone large mass. The stagnant layer still tends to insulate the tube fromthe main body of the liquid, but the mixing promotes heat transfer bykeeping a larger temperature differential between the stagnant layer andthe next liquid layer. In either case, anything which disturbs thestatic layer or promotes more mixing helps to reduce the insulatingeffect within the tube, and therefore increases the rate of heattransfer.

After heat is transferred from the main body of liquid to the stagnantlayer, it has to flow across the stagnant layer. Then, heat has to flowacross the interface between the stagnant layer and the tube innersurface. Any interface provides some resistance to heat flow. After heatflows to the tube inner surface, it has to flow through the tube to thetube outer surface. To facilitate this heat flow, heat transfer tubesare usually made out of a material which readily conducts heat, or aheat conductor. Copper is one material which is considered to be a goodconductor of heat. Then, the heat flows across the interface between thetube outer surface to any liquid contacting the outer surface There canbe heat flow across a stagnant liquid layer on the tube outer surface,and then the liquid absorbs the heat and boils. The boiling liquidabsorbs a specific amount of heat, called the heat of vaporization, tochange from the liquid state to the vapor state.

An interface between the fins and the tube exists if the fins areconstructed separately from the tube, and then attached. This is true ifthe fin and tube are constructed of the same material, such as copper,or from different materials. Any interface causes some resistance toheat flow. If the fins are formed from the tube wall, there is nointerface and heat flow is improved. In this discussion, fins formedfrom the tube wall are referred to as being monolithic with the tube,and it is preferred that fins be monolithic with a tube to minimizeresistance to heat flow.

The tube should be made from a malleable substance so the fins can beformed from the tube without cracks or breaks forming in the tube wall.Cracks or breaks limit the structural integrity and strength of a tube,and can also provide resistance to heat flow. Generally these tubes areused in shell and tube heat exchangers, and the ends of the tubes areaffixed in tube sheets of the heat exchanger. A malleable tube can beeasier to install in a heat exchanger tube sheet. The tube should alsobe constructed from a material which readily conducts heat. Copper isoften used in tube construction because of its malleability and heatconducting

Special Evaporation Principles

Evaporation tubes have specific design features which are different thanthose features preferred for a condensation tube. Evaporation tubes aretypically immersed in the liquid to be evaporated, so the tube outersurface is constantly wet. Factors which can enhance evaporation includeproviding a nucleation site for the initial formation of bubbles,providing enclosed areas where liquid can be superheated, and providingholes or access ports to the enclose areas where vapor can escape andmore liquid can be introduced.

Nucleation sites for boiling are often very small imperfections or sharppoints on the boiling surface. An enclosed area on a tube provides for arelatively small quantity of liquid to be essentially surrounded by heattransferring surfaces from the finned tube, so the amount of heattransfer surface area per volume of liquid is large. This allows for theliquid to be rapidly heated to facilitate boiling or vaporization. Thiscan result in the liquid being temporarily superheated, which is whenthe temperature of the liquid is greater than the liquid's boilingtemperature. Vapors are less dense than liquids, so when a liquidvaporizes it expands. If the vaporizing liquid is enclosed, it producespressure as it vaporizes. Vapors also expand as they are heated, soheating of a vapor in an enclosed area also increases pressure.

Small holes in the enclosed area allow for the small quantity of liquidto escape after is has vaporized, and the pressure from vaporizationtends to push the vapor out of the hole. Normally, surface tension wouldreduce liquid flow through small holes, unless there is a large enoughpressure difference to force or push the liquid through the hole. Theescaping vapor leaves a reduced pressure in the enclosed area, whichdraws liquid in through the small holes after the vapor has escaped, andthe process repeats. This serves as a sort of pumping action, whereliquids are drawn into enclosed area, vaporized, and pushed out of theenclosed areas.

The small hole in the enclosed area has to be small enough to prevent aliquid from freely flowing through the hole. The small holes can be onecontinuous hole, as long as it serves to prevent the liquid from freelyflowing into the enclosed area. To prevent a liquid from flowing througha continuous hole or a series of small holes, there must be a hole gapsmall enough that the liquid surface tension prevents the liquid frompassing. Reference in this description to several small holes isintended to include one long hole with a gap small enough to preventliquid from flowing through, such that the long hole serves the samefunction as several small holes. The long hole serves essentially asseveral small holes which are connected together.

Finned Tube Main Body

One embodiment of the finned tube 10 of the current invention is shownin different perspectives in FIGS. 1, 2 and 3. This discussion focuseson the embodiment shown, but this discussion is not intended to belimiting. Other embodiments are possible, and will be apparent to oneskilled in the art.

The tube 10 includes a main body 12 which has an outer surface 14 and aninner surface 16. The main body 12 is the base for any shapes orstructures on the outer or inner surface 14, 16. This main body 12should be made of a material which conducts heat readily. Metals aregenerally good conductors and are frequently used for the constructionof tubes of the current invention. Copper is a particularly common metalused for tube 10 construction, but aluminum, other metals, variousalloys and even non-metallic materials are also possible. The materialshould also be malleable or formable such that the various structures onthe inner and outer surface 14, 16 can be formed without damaging theintegrity of the tube body 12. This allows for the structures to beformed from the tube body 12, which results in the structures beingmonolithic with the tube body 12.

Tube Fins

The tube 10 has at least one fin 20 formed on its outer surface 14. Thefin 20 generally protrudes or extends circumferentially from the tubebody outer surface 14, and is usually helical. The tube 10 often has afirst end 22 and a second end 24 without any fins 20 which facilitatesforming a seal between a tube end 22, 24 and a heat exchanger tubesheet. These ends 22, 24 are generally smooth. There is typically sometransition area between the smooth ends 22, 24 and the finned portion ofthe tube 10.

It is possible that one single fin 20 is helically wound around theentire length of the finned portion of the tube 10. It is also possiblethat there will be a plurality of fins 20 helically winding around thetube 10. In either case, when looking at a section of the tube bodyouter surface 14, it will appear as though there are several adjacentcircumferential fins 20 protruding from the tube body outer surface 14.When viewed along the axial direction of the tube 10, fin 20 sectionsnext to each other are referred to as adjacent fins 20, despite the factthat they might be the same fin 20 helically wrapping around the tubebody outer surface 14. The fin 20 is formed from the material of thetube body 12, so the fin 20 is monolithic with the tube body 12.

Each fin 20 has several parts including a fin base 26, a fin top 28, anda fin side wall 30. The fin base 26 is at the point where the fin 20connects to the tube body outer surface 14. The fin top 28 is oppositethe fin base 22 and is the highest point of the fin 20 relative to anaxis of the tube 62. The fin side wall 30 includes a left side wall 32and a right side wall 34 opposite the left side wall 28. A channel 36 isdefined between two adjacent fins 20 over the tube body 12 such that thechannel 36 is between a right side wall 34 of one fin 20 and a left sidewall 32 of an adjacent fin 20. The fin 20 can be approximatelyperpendicular to the tube body 12 such that the fin 20 extendsessentially straight out from the tube body outer surface 14. In such acase, the fin 20 would extend radially from the tube 10. It is alsopossible for the fin 20 to be positioned at other angles to the tubebody outer surface 14.

The fin 20 also has a fin upper portion 38, which is deformed or moldedfrom the fin 20 to form a roof 40 over the channel 36. The fin upperportion 38 can be split to extend both left and right of the fin 20, asin FIG. 3, or the fin upper portion 38 can be deformed in just onedirection from the fin 20, as shown in FIGS. 4 and 5. The roof 40 doesnot have to completely cover the channel 36, and there should be holes42 or pores 42 defined by and penetrating the roof 40 to into thechannel 36. Notches 43 can be formed in the fin top 28 to define theholes 40, but the holes 40 can be formed in other ways as well. The roof40, the left and right fin side walls 32, 34, and the tube body outersurface 14 define a boiling cavity 44, which is basically an enclosedchannel 36.

The boiling cavity 44 is very effective at promoting boiling orevaporation on the tube exterior. Liquid in the boiling cavity 44 issurrounded on four sides by tube surfaces which transfer heat to theliquid. The tube surfaces facing the liquid in the boiling cavity 44 arethe tube outer surface 14, the left and right fin side walls 30, 32, andthe roof 40. A liquid droplet in the boiling cavity 44 has a relativelylow volume with a relatively high surface area in contact with theboiling cavity surfaces 14, 30, 32, 40, which results in the liquidbeing heated rapidly. As the liquid boils and turns into a gas, itexpands and increases the pressure inside the boiling cavity 44. Thisforces the boiled gas out through a roof hole 42, and the exiting gasleaves a partial vacuum or low pressure inside the boiling cavity 44.

The partial vacuum inside the boiling cavity 44 facilitates motion. Thepartial vacuum pulls and moves drops of liquid inside the boiling cavity44 which were next to the exiting gas into the location where theexiting gas was, which serves to agitate the moving liquid drop andthereby promote heat transfer. Also, when the vaporized gas exits theboiling cavity 44, the low pressure produced pulls more liquid fromsomewhere along the outside of the cavity 44 through a roof hole 42 intothe cavity 44. Without the low pressure in the boiling cavity 44,surface tension would tend to prevent liquids from readily passingthrough the roof holes 42 into the boiling cavity 44, so the pressurehas to be enough to overcome the liquid surface tension. This action ofmoving and mixing liquids inside the boiling cavity 44 combined withpushing out vaporized gas and pulling in additional outside liquids isreferred to as a pumping action, and it greatly increases the rate ofheat transfer and vaporization of liquids.

The fin 20 can be deformed or shaped in a wide variety of ways, such asforming wings or side fins (not shown) extending from the fin side wall30 below the roof 40 to form an upper and lower boiling cavity 44. Manydifferent boiling or evaporation fin 20 configurations are possiblewithin the current invention. The size of the boiling cavity 44 and theroof holes 42 can be varied, and specific sizes are more efficient forcertain compounds. For example, if the tube 10 were to used for therefrigerant R22, different sized holes 42 and boiling cavity 44dimensions would be employed than if the tube 10 were to be used for therefrigerant R123.

Inner Surface Ridges

Heat transfer across the tube 10 can be improved by providing bettertransfer of heat between the tube body inner surface 16 and a liquidwithin the tube 10. A ridge 50 or a plurality of ridges 50 can bedefined on the tube body inner surface 16 to help facilitate more rapidheat transfer, and these ridges 50 can be monolithic with the tube body12. The ridges 50 on the inner surface 16 are generally helical and havea depth 52 and a frequency or pitch. The frequency is the number ofridges 50 within a set distance. The ridges 50 are also set at differentcut angles relative to the tube axis 62. There can be several ridges 50formed within the tube 10, and the number of ridges 50 allows for apredetermined cut angle, ridge depth 52, and frequency. The number ofridges 50 is referred to as the number ridge heads or the number ofridge starts.

The depth 52 and the frequency of the ridges 50 can vary, and the cutangle can be set to cause the cooling liquid to swirl within the tube10. A swirling liquid tends to increase heat transfer by increasing theamount of agitation within the cooling liquid. Agitation tends tominimize or eliminate the layers of fluid in laminar flow, and agitationalso tends to minimize the thickness of the stagnant layer of fluid nextto the tube inner surface 16. Additional measures which can inducevortexes and local agitation at or very near the tube inner surface 16further reduce the stagnant layer of fluid next to the tube innersurface 16, and thereby increase heat transfer. However, variations and3-dimensional contours or texture also tend to increase the resistanceto flow within a tube 10, so more pressure is required to push a givenamount of fluid through a pipe in the same amount of time. A largerridge depth 52 and a smaller ridge frequency tend to increase the rateof heat transfer, but they also increase the resistance to flow insidethe tube 10.

Tube Indentations

Referring now to FIG. 6, the tube 10 includes indentations 60 definedand depressed into the outer surface 14 such that the inner surface 16protrudes or extends towards the tube axis 62 directly opposite theindentation 60 in the outer surface 14. This basically means theindentation 60 goes through the tube body 12 and pushes the innersurface 16 inward. This inner surface protrusion is referred to as a rib64, which is the inner surface 16 counterpart to the outer surface 14indentation 60. The indentation 60 loops around the tube 10, and theloops can be in a variety of forms. For example, the loops can behelical as shown in FIG. 7, or they can be a plurality of radialindentations 60 which form successive rings around the tube 10 as shownin FIG. 8, or they can be a plurality of indentations 60 helicallylooping around the tube 10 in opposite directions as shown in FIG. 9.Other indentation 60 patterns are possible, and are within the scope ofthe current invention.

Referring now to FIGS. 3 and 6, the ribs 64 on the inner surface 16affect fluid flow patterns inside the tube 10. The rib 64 projects intothe tube 10, and fluid flowing over the rib 64 tends to form vortexesand eddies downstream from the rib 64. These vortexes decrease thestagnant fluid layer next to the inner surface 16, and thereforeincrease the heat transfer rate inside the tube 10. The rib 64 shouldnot form a barrier which significantly impedes flow through the tube 10,so the rib 64 cannot have a rib height 66 greater than a tube innerradius 68. The rib height 66 is the height from the top of the rib 60 tothe tube inner surface 16. Preferably, the ratio of the rib height 66 toa nominal tube outside diameter 70 is between 0.02 and 0.2, where thenominal tube outside diameter 70 is measured from the fin tops 28.

A helical rib 64 and the corresponding indentation 60 can wrap aroundthe tube 10 either in the same direction as the internal ridges 50, orin the opposite direction of the internal ridges 50. If the rib 64 wrapsin the same direction as the ridges 50, the rate of heat transfer andthe resistance to flow is not increased as much as if the rib 64 wrapsthe opposite direction as the ridges 50. The ridges 50 induce a swirlingflow direction, and the rib 64 also induces a swirling flow direction,as long as both are helical. When the rib 64 spirals counter to theridge 50, the change in induced flow direction between the rib 64 andthe ridges 50 accounts for the greater heat exchange rate and resistanceto flow. The double helical indentation 60, and thus the double helicalrib 64, is particularly effective when the inside liquid flow rate isrelatively low, and discontinuous rib 64 belts are particularlyeffective when the inside liquid flow is laminar. The rib direction andheight 66 can be set to keep the tube resistance to flow within a 1.5fold increase of the resistance to flow without the rib 64.

Tube Use in a Heat Exchanger

The tube 10 is often used in a heat exchanger 72, as shown in FIGS. 1and 10. The tube first and second ends 22, 24 are fixed to two tubesheets 74 with a tube side inlet 76 and a tube side outlet 78 for fluidflow through the tube 10 interior. The tubes 10 are contained inside ashell 80. In the example shown, the heat exchanger 72 is a dual passheat exchanger 72 with fluid entering and exiting from the same side ofthe heat exchanger 72 for flow through the tubes 10. There is also ashell side inlet 82 and a shell side outlet 84 for flow past the outsideof the tube 10. The heat exchanger 72 shown is a drip exchanger, withshell side fluid being collected at the bottom of the shell 80 andrecirculated to a spray device 86 positioned above the tube 10 by a pump88.

A drip exchanger 72 or drip evaporator 72 uses less refrigerant than aflooded evaporator. The flooded evaporator has the shell 80 filled withliquid refrigerant such that the tubes 10 are immersed in liquid, butthe drip evaporator 72 is mostly filled with gas or vapors. The dripevaporators 72 tend to be more energy efficient than the flooded type,and they use less refrigerant. Often the refrigerants used arechlorofluorocarbons (CFCs) or hydra chlorofluorocarbons (HCFCs), whichhave many regulations controlling their use, so means of using lessrefrigerants are desirable.

For an evaporator tube 10 to function most efficiently, the outersurface 14 (or the evaporating surface, which can be the inner surface)should be kept wet with liquid to be evaporated. With a flooded heatexchanger, the tube outer surface 14 is immersed in liquid, so keepingthe surface wet is not a consideration. With a drip evaporator, therecan be portions of a tube outer surface 14 which are not wet, and whichtherefore cannot evaporate any liquids. The way a liquid flows over atube 10, the type of spray device 86 used to distribute the liquid, andother factors can affect the rate of evaporation. It has been noted thattubes 10 with indentations 60 tend to keep more of the tube outersurface 14 wet than tubes 10 without indentations 60. This may bebecause the indentations 60 serve to collect and re-distribute theliquid along the tube 10, or because the indentations 60 allow foreasier liquid access to the tube boiling cavities, or it may be due toother reasons. Whatever the reason, measurements of overall heattransfer rates with tube indentations 60 have shown increases of up to20% over heat transfer rates for tubes 10 without indentations 60.

A heat exchanger 72 with tubes 10 that are 20% more efficient canutilize tubes 10 that are 20% shorter, or fewer tubes 10 with largerdiameters can be used. By changing the heat exchanger 72 design,pressure drop issues from tubes 10 with a higher flow resistance can beaddressed. It should be noted the tubes 10 of the current invention alsocan be used in flooded type evaporators. The internal rib increases thetube internal heat exchange rate, which can benefit the tube overallheat exchange rate.

Tube Forming Process

Finned tubes 10 are generally formed from relatively smooth tubes 10with a tube finning machine, which is well known in the industry. Thetube finning machine includes an arbor 90 as seen in FIGS. 11 and 12,with further reference to FIG. 3. Frequently, a tube finning machinewill include three or more arbors 90 positioned around the tube 10, sothe tube 10 is held in place by the arbors 90. The arbors 90 arepositioned and angled such that each complements the others. A tube 10is provided and fed through the finning machine such that a tube wall 92is positioned between the arbor 90 and an inner support 94. The arbor 90deforms the tube outer surface 14, and the inner support 94 can deformthe tube inner surface 16. Actually, the arbors 90 hold various tools ordiscs, and the tools contact and shape the tube outer surface 14, so thearbors 90 serve as a form of tool holder. The tube wall 92 is generallyrotated relative to the arbor 90 and moves axially with the innersupport 94 as it rotates.

The arbor 90 generally includes several fin forming discs 96 whichsuccessively deform the tube wall 92 to form one or more helical fins 20on the tube outer surface 14. Successive filming discs 96 tend toproject deeper into the tube wall 92 such that fins 20 are formed andpushed upwards by the finning discs 96. The inner support 94 can includerecesses 98 such that helical ridges 50 are formed on the tube innersurface 16 as fins 20 are formed on the tube outer surface 14.

After the fin forming discs 96 have formed the fins 20, various otherdiscs can be included on the arbor 90 to further deform and defineaspects of the final tube 10. There are a wide variety of discs whichcan be included to produce many different shapes, include a boilingcavity 44. One of many examples is shown. After the fin forming discs96, a notching disc 100 notches the fin top 28. The notching disc 100has teeth which press into the fin 20 to form the notch 43. In FIG. 11,a portion of the fin 20 is shown in front of the notching disc tooth,and in FIG. 12, the tube wall 92 is shown sectioned such that there isno material shown in front of the notching disc tooth forming the notch43. The notch 43 becomes a hole 42 in the roof 40 as the fin 20 isfurther deformed. The fin splitting disc 102 splits the fin top 28.Then, a flattening disc 104 flattens the fin tops 28 to form the roof40. The roof 40 can be formed with a small gap between adjacent fin tops28 to produce the hole 42, or other methods can be used to produce thehole 42.

After the fins 20 and the inner ridges 50 are formed, the indentation 60is produced in a subsequent step, as shown in FIG. 6. An indentationdisc 106 is rolled over the tube 10. The indentation disc 106 cutsthrough the fin 20 and the boiling cavity 44, and presses inward on thetube inner surface 16 to form the rib 64. The indentation disc 106 cutsinto and interrupts the boiling cavity 44, and can provide relativelyeasy access by a fluid to the boiling cavity 44 at discrete locations.

This is one example of how the various deformations of the originalrelatively smooth tube 10 are produced. There are other possible ordersand designs of discs and tools which can be used as well.

EXAMPLE DIMENSIONS

The dimensions of the current invention can vary, but example dimensionsare provided below which will give the reader an idea as to at least oneembodiment of the current invention.

The inter-fin distance is the distance between a center point of twoadjacent fins 20 and this distance can be between 0.3 and 0.7millimeters.

The fin 20 has a thickness between the left and right side wall 32, 34,and this thickness can be between 0.1 and 0.5 millimeters.

The fin 50 has a height measured from the fin base 22 to the fin top 24,and the fin height can be between 0.3 and 1.5 millimeters.

The ridge 74 formed on the tube body inner surface 16 has a depth 52,and this depth can be between 0.1 and 0.5 millimeters. The internalridge angle with the axis 62 can be set at 46°, and the ridge starts canvary between 8 and 50.

The hole 64 defined in the barrier 58 can have an area between 0.01 and0.2 square millimeters.

The tube wall 92 thickness can vary between 0.75 and 3 mm.

The indentations 60 have an indentation depth and a pitch, wherein thepitch is the distance between two adjacent indentations measured axiallyalong the tube 10. The indentation depth can be a ratio of the nominaltube outside diameter 70, and the ratio can be between 0.02 and 0.2,with the indentation depth ranging between 0.25 to 7 mm. The indentationpitch can a ratio with the nominal tube outside diameter 70, and thisratio can be 0.25 to 2, with the pitch ranging between 3 and 75 mm.

The nominal tube outside diameter 70 can range between 12 and 39 mm.

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed here.Accordingly, the scope of the invention should be limited only by theattached claims.

1. A finned tube comprising: a tube body having an outer surface, aninner surface, and an axis; at least one monolithic fin defined on thetube body outer surface such that, in the axial direction, the tubeouter surface has adjacent fins, wherein a channel is defined betweenadjacent fins, and wherein the fin has an upper portion deformed to forma roof over the channel, and wherein the roof defines holes penetratingin to the channel; a plurality of helical ridges defined on the tubebody inner surface; at least one helical indentation defined in the tubebody outer surface such that the indentation extends the tube body innersurface towards the tube axis.
 2. A finned tube comprising: a tube bodyhaving an inner surface; a helical ridge defined on the tube body innersurface; and an indentation defined in the tube body, the indentationlooping around the tube.
 3. The finned tube of claim 2 wherein theindentation is helical, and wherein the indentation spirals around thetube in the opposite direction of the ridge.
 4. The finned tube of claim2 wherein the indentation is helical, and wherein the indentationspirals around the tube in the same direction as the ridge.
 5. Thefinned tube of claim 2 wherein the indentation further comprises aplurality of indentations forming successive rings around the tube. 6.The finned tube of claim 2 wherein the indentation further comprises aplurality of helical indentations spiraling around the tube body inopposite directions.
 7. The finned tube of claim 2 wherein the tube bodyincludes an outer surface, the finned tube further comprising amonolithic helical fin defined on the tube body outer surface such thatthe tube body outer surface includes adjacent fins when viewed axially,wherein a channel is defined between adjacent fins, and wherein the finincludes an upper portion deformed to form a roof over the channel. 8.The finned tube of claim 7 wherein the tube body outer surface, adjacentfins, and the roof define a boiling cavity, and wherein the roof definesa plurality of roof holes penetrating into the boiling cavity.
 9. Thefinned tube of claim 8 wherein the fins include a fin top havingnotches, and the notches define the roof holes.
 10. The finned tube ofclaim 2 wherein the tube is comprised of copper.
 11. A finned tubecomprising: a tube body having an outer surface; a monolithic findefined on the outer surface such that the outer surface includesadjacent fins viewed in the axial direction, wherein a channel isdefined between adjacent fins, and wherein the fins include an upperportion deformed into a roof over the channel; and an indentationdefined in the tube body, the indentation looping around the tube. 12.The finned tube of claim 11 wherein the indentation is helical.
 13. Thefinned tube of claim 11 wherein the indentation further comprises aplurality of helical indentations spiraling around the tube in oppositedirections.
 14. The finned tube of claim 11 wherein the indentationfurther comprises a plurality of indentations forming successive ringsaround the tube body.
 15. The finned tube of claim 11 wherein the tubebody further includes an inner surface, the finned tube furthercomprising a helical ridge defined on the tube body inner surface. 16.The finned tube of claim 15 wherein the indentation is helical and theindentation spirals around the tube in the opposite direction as theridge.
 17. The finned tube of claim 15 wherein the indentation ishelical and the indentation spirals around the tube in the samedirection as the ridge.
 18. The finned tube of claim 15 wherein the tubebody outer surface, adjacent fins, and the roof define a boiling cavity,and the roof defines a plurality of holes penetrating into the boilingcavity.
 19. The finned tube of claim 18 wherein the indentationinterrupts the boiling cavity.
 20. The finned tube of claim 11 whereinthe tube is comprised of copper.